This invention relates to nondestructive evaluation of contoured objects such as gas turbine wheel dovetail posts and, more particularly, to a backing piece, an inspection arrangement, and a method for their use in conjunction with eddy current inspection of a contoured surface, such as a gas turbine wheel dovetail post surface.
High performance gas turbine engines typically have a compressor and a turbine which each include a stator and a rotor. The stator is comprised of axially-spaced annular banks having circumferentially arranged, fixed stator vanes. The rotor is comprised of axially-spaced rotatable wheels of circumferentially arranged rotor blades positioned between the annular banks of fixed stator vanes. Each rotor blade typically comprises an outer rotor tip, an air foil and an inner dovetail-shaped base or root which matably mounts within a complementarily-shaped, axial dovetail slot formed between adjacent ones of the dovetail posts on the rim of a rotor wheel. Examples of gas turbine engines include, but are not limited to, gas-turbine power-generation equipment and gas-turbine aircraft engines.
Nondestructive evaluation of gas turbine wheel dovetail posts for the presence of cracks has previously been accomplished by scanning the surface of the dovetail post on either side of the dovetail slot using a single inflexible coil probe. More recently, the instant assignee has developed, a flexible eddy current array probe for use in dovetail post surface inspections in gas turbine aircraft engine parts. The key to using the flexible eddy current array probe is the contact pressure between the probe and the dovetail post surface being inspected. Even or uniform contact pressure must be maintained over the entire area of the flexible eddy current probe during the inspection process.
The dovetail posts employed in gas turbine aircraft engine parts are considerably smaller in dimension when compared to dovetail posts employed in power generation gas turbines. With respect to gas turbine aircraft engines, a pneumatic backing system is used to ensure that the desired contact pressure is provided between the flexible eddy current probe and the inspection surface of the dovetail post. A problem exists, however, in that while this approach works well for the small dovetail posts of gas turbine aircraft engines, it is not efficient for the large dovetail posts used in power generation gas turbine components since the large size of the dovetail posts would require time-consuming multi-passes of the inspection probe during the inspection process.
Consequently, a need exists for an innovation which will provide a solution to the aforementioned problem without introducing any new problems in place thereof.
A contoured surface eddy current inspection backing piece, inspection arrangement, and inspection method are designed to satisfy the aforementioned need. The backing piece, inspection arrangement and inspection method are adapted for effectively carrying out eddy current inspection, generally, of any contoured surface and, more particularly, of the surface of a gas turbine wheel dovetail post of a power generation gas turbine.
In one embodiment of the invention, a backing piece is provided for use in eddy current inspection of a contoured surface, such as a turbine wheel dovetail post surface. The backing piece comprises a body made of a resiliently yieldable material so as to be flexible and capable of expansion in size. The body has at least one contoured exterior surface that conforms in shape to a contoured surface of a workpiece to be inspected. The body further has means therein to facilitate expansion of the body, and to provide sufficient contact pressure between the contoured exterior surface of the body and the contoured surface of the workpiece to enable the contoured surface of the workpiece to be inspected. The body is substantially solid, and made of a material which, for example, can be a castable rubber compound. The expansion facilitating means is an elongated slot formed in the body and open at a location on a side of the body displaced from the contoured exterior surface thereof.
In another embodiment of the invention, an inspection arrangement provides for eddy current inspection of a contoured surface of a workpiece, such as the dovetail post surface of a turbine wheel. The inspection arrangement comprises the flexible and expandable backing piece, as described above, and a flexible eddy current array probe attached to the at least one contoured exterior surface of the backing piece such that the probe faces the contoured surface of the workpiece to be inspected when the backing piece is positioned adjacent to the workpiece. The inspection arrangement further comprises at least one shim insertable into the slot in the backing piece for the purpose of expanding the backing piece in size to provide sufficient contact pressure between the probe and the contoured surface of the workpiece to enable the contoured surface of the workpiece to be inspected when the backing piece is positioned adjacent to the workpiece.
In still another embodiment of the invention, a method is provided for eddy current inspection of a contoured surface of a workpiece, such as the dovetail post surface of a turbine wheel. The inspection method comprises the steps of forming a backing piece of resiliently yieldable material which is flexible and capable of expanding in size and having at least one contoured exterior surface conforming in shape to the contoured surface of the workpiece to be inspected, attaching a flexible eddy current array probe to the at least one contoured exterior surface of the backing piece such that the probe faces the contoured surface of the workpiece to be inspected when the backing piece is disposed adjacent to the workpiece, and expanding the backing piece to provide contact pressure between the probe and the contoured surface of the workpiece to be inspected. Further, the expanding step includes forming a slot in the backing piece so as to be open at a location on a side of the backing piece displaced from the at least one contoured exterior surface of the backing piece, and inserting at least one shim into the slot in the backing piece so as to expand the backing piece in size to provide sufficient contact pressure between the probe and the contoured surface of the workpiece to enable the contoured surface of the workpiece to be inspected.